1. Field
The present invention relates to processing information regarding weapon fire, and, more particularly, to systems and methods of processing information regarding weapon fire, such as determining weapon fire location using projectile shockwave and muzzle blast time(s) of arrival data.
2. Description of Related Information
Some gunshot detection systems use muzzle-blast arrival times to locate gunshots. They apply multilateration techniques to solve for a shot time and location to best match the pulse arrival times. For each pair of sensors, the speed of sound and difference between arrival times define a hyperbola representing the locus of potential shooter locations. In a hypothetical situation with no sensor location or time measurement error and an exactly known, uniform speed of sound, the actual shooter location would be the geometric intersection of the hyperbolas defined by each combination of sensor pairs. In real application, the hyperbolas do not intersect perfectly, so a least-squares minimization is used to find the best fit for all measured data. Such a system can be highly accurate when widely dispersed sensor arrays encompass the coverage area. For a shooter outside the array, however, fewer sensors tend to detect the shot, decreasing the probability of having enough detections to compute a location. Even with a sufficient number of detections to attempt to find hyperbolic intersections, the arms of the hyperbolas in the direction of the shooter tend toward parallel. With even small amounts of measurement error, the expected intersection point becomes indistinct, resulting in poor location accuracy, particularly in range estimation.
Better accuracy can be obtained for muzzle-blast detections outside the array when the sensors report a direction of arrival in addition to the time of arrival. With the bearing to the shooter known from at least two locations, the problem can go from multilateration to much simpler triangulation, or a hybrid approach that combines elements of both. A directional sensor typically requires a small array of microphones with fixed relative locations. The absolute orientation of this small array must be known in order to convert from a relative arrival direction to an absolute arrival direction. Such a sensor is significantly more complex and expensive than a single-microphone sensor. This is particularly true for mobile sensors, as not only the position, but also the orientation, must be continuously updated. The audio processing capability of such a sensor must also be greater than that for a single-microphone sensor, not only because it handles multiple audio channels, but also because high sampling rates are required to ensure adequate precision in time-of-arrival measurement at each microphone. Without sampling rates well above the threshold of human hearing, a directional sensor must be large enough that portability will be adversely affected, or else it will suffer from poor bearing accuracy. When the breadth of an array of directional sensors is small compared to the distance to the shooter, there can still be a great deal of error in the resulting range estimates due once again to the near-parallel bearings.
One means of obtaining a better range estimate, assuming a sensor is under fire or near the line of fire, is to use information from detected bullet shockwaves. A single high-frequency multi-microphone directional sensor can process acoustic shockwave arrival data to reconstruct the shape, duration, and intensity of the shockwave as it passes the sensor. By combining this reconstruction with models of ballistics and shockwave propagation, the bullet caliber and trajectory can be estimated. The caliber can then be used to select a model of the bullet's external ballistics, which leads to a good estimate of bearing and range. This estimate can be further improved if combined with the arrival time and direction of the following muzzle blast, when available.
FIG. 1 shows the bullet-shockwave and muzzle-blast phenomena. As the bullet initially leaves the barrel, a muzzle blast is created by the rapidly expanding gases behind it. This blast, shown as a circular dot-dashed curve, expands outward from the muzzle at the speed of sound. As it passes each sensor, it is detected as an impulse. Meanwhile, as the bullet travels ahead of the muzzle blast at supersonic speed, it produces a shockwave that expands outward from the bullet path at the speed of sound. The direction of the expanding shockwave at any point along its leading edge is determined by the bullet's Mach number at the instant that portion of the wave was created. If the bullet passes near a sensor while still supersonic, the sensor detects the shockwave as a very sharp impulse.
There have been a number of important recent advances in the ability to cheaply and accurately locate portable devices. Some of these include the widespread availability of GPS, compact INS units, and various means of photogrammetry. These advances have been accompanied by the proliferation of small, mobile computing devices such as those in notebook or tablet computers and cell phones. Many such devices incorporate microphones and wireless communication capabilities along with general-purpose computers capable of altering and enhancing their function through the use of installable software. Such mobile devices running appropriate software could be harnessed as part of a gunshot detection system.
An exemplary implementation of such a system might consist of a group of three or more soldiers, outfitted with these devices, which we'll call sensors, taking fire from an unknown sniper position as shown in FIG. 1. Standard tactics in a combat situation would require separation between individual soldiers on the order of ten meters. The relative sensor locations could easily be known to accuracy on the order of centimeters or decimeters using various technologies. Each sensor would have at least one general-purpose microphone capable of detecting impulsive events such as bullet shockwaves or muzzle blasts. Those skilled in the art would recognize that in such a situation, multilateration using detected muzzle-blast impulse arrival times would result in very low range accuracy. This is due to the source being well outside the array in combination with a high ratio of sensor position error to array size. Similarly, attempts to use the bullet-shock impulse arrival times to reconstruct the shape of the shockwave would at best be cumbersome and at worst be inaccurate. This is because the relative positions of the microphones are imprecisely known (e.g., known only to a precision which is a substantial portion of the inter-sensor distances, etc.), as they usually are in such an application, and also because the large separation between sensors prevents use of an assumption that all detected shocks are created by a constant-speed bullet.
As such, among other drawbacks overcome via the innovations herein, there is a need in such circumstances for systems and methods of processing the arrival times of both the projectile shock and the muzzle impulse to obtain an optimal solution of azimuth, elevation, and/or range.